Flags related to vtags

As mentioned earlier, the usage of vtags is not manditory. Therefore we define two flags values that can be used to control the behavior of the calls with respect to vtags:

TODO: pick a reasonable prefix for our flags.

• FLAG_VTAG: Indicates that the vtag is valid. The caller does not have a valid vtag for input, nor does he desire a valid vtag in response.
• FLAG_VTAG_RETURN: Indicates that the caller wishes to obtain a vtag from the operation. However, the caller does not wish to use a vtag for input.

By default calls ignore vtag values on input and do not create vtag values for output.